Presently, it is not uncommon for vehicles, such as trucks and buses, to be equipped with both service brakes and an exhaust brake. The use of an exhaust brake in combination with service brakes may lighten the load, so to speak, for the service brakes, thereby increasing the life and decreasing the maintenance cost of the service brakes.
Exhaust brakes have been known for quite some time. Many exhaust brakes, such as the brake disclosed in Okada et al., U.S. Pat. No. 4,748,812 (issued Jun. 7, 1988) for a Turbo Compound Engine, are actuated by a fluid driven actuator. Fluid driven actuators, however, require the maintenance of a special fluid reservoir and hydraulic system dedicated to the exhaust brake. Since most exhaust brakes need only have two positions, fully open and fully closed, complicated and expensive hydraulic actuators may not be required.
Other exhaust brakes, such as the one disclosed in Clarke et al., U.S. Pat. No. 5,445,248 (issued Aug. 29, 1995) for an Exhaust Brake, may use a pneumatic actuator for the exhaust brake. The pneumatic actuator may open and close a valve, such as a butterfly valve, in the brake. The pneumatic actuator in the Clarke patent comprises a piston and chamber. The piston is drawn in and out of the chamber by varying the pressure within the chamber. The drawing in and out of the piston is, in turn, used to open and close a circular butterfly valve just downstream of an exhaust brake elbow.
Pneumatic actuators for an exhaust brake may be driven by a vacuum supply included in the vehicle in which the exhaust brake is installed. The vacuum supply used to drive the actuator may be the same supply used for control, assistance, and/or activation of the service brakes of the vehicle (as well as other vacuum actuated subsystems, such as heating and air conditioning). Due to the limits of the pump used to provide the vacuum supply, the combined vacuum requirements of the exhaust brake and service brake may exceed the vacuum capacity of the vacuum supply. Because the service brakes provide a much greater portion of the vehicle braking power, the service brakes must be given priority over the exhaust brake should the vacuum supply be deficient for both types of brakes.
The aforementioned vacuum deficiency problem may be particularly acute when the vehicle is operated at higher altitudes. The operation of the vehicle at higher altitudes decreases the vacuum capabilities of the vehicle's vacuum pump. The higher the altitude, the lower the strength of the vacuum produced by like powered pumps. High altitudes also increase the pull down time of the pump; i.e. the time it takes the pump to achieve a particular level of vacuum starting from ambient pressure.
Systems have been devised to control the operation of combination exhaust brakes and service brakes. Tezuka et al. U.S. Pat. No. 5,516,200 (issued May 14, 1996) for a Controller for Tractor Trailer Braking Devices, discloses an integrated system for controlling the operation of service brakes power assistance, an exhaust brake, a tractor retarder brake, and a trailer retarder brake. The Tezuka system enables the various vehicle braking subsystems to be selectively interlocked so that activation of one braking subsystem may be linked to the activation of another braking subsystem. The Tezuka system does not address the control problems which arise from use of service brakes and exhaust brakes activated or assisted by a common vacuum supply.
The prior art references of which the applicant is aware do not even recognize the broader safety problems which may arise as a result of having service brakes and exhaust brakes connected to any type of common pneumatic or hydraulic actuation system. Leaks or malfunctions in the exhaust brake part of the actuation system could result in interference with the service brake assist part of the actuation system. Since the service brakes are usually far more effective in braking the vehicle, interference with the activation of the service brakes needs to be avoided if possible.
Accordingly, the Applicant has determined the necessity of providing a system for controlling the connection of an exhaust brake actuator with an actuation system that is also connected to the service brakes. The control of the exhaust brake connection may be responsive to the ability of the actuation system to supply ample actuation vacuum for both the exhaust brake and assistance for the service brakes. If ample actuation is lacking, the service brakes assistance may be prioritized and the exhaust brake disconnected from the actuation system. Lack of ample actuation may be caused by reduced performance of the driving force of the actuation system, or by the development of leaks or disconnection in the elements connecting the actuation system and the exhaust brake.
More specifically, the Applicant has discovered the necessity of a system for controlling the connection of an exhaust brake actuator with a vacuum supply. The control of the exhaust brake connection may be responsive to the level of vacuum, the pull down time of the vacuum pump, or the vacuum flow rate. The system may preferably also provide a safety mechanism which isolates the vacuum supply from the hose connecting the vacuum supply and the exhaust brake actuator should the hose or any other component in the system fail or leak.